Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. OCT is a method of interferometry that uses light containing a range of optical frequencies to determine the scattering profile of a sample. The axial resolution of OCT is inversely proportional to the span of optical frequencies used. OCT technology has found widespread use in ophthalmology for imaging different areas of the eye and providing information on various disease states and conditions. Commercial OCT devices have been developed for imaging both the anterior and posterior sections of the eye (see for example Cirrus HD-OCT, Visante Omni, and Stratus (Carl Zeiss Meditec, Inc. Dublin, Calif.)). The Cirrus HD-OCT system allows for imaging both the anterior and posterior regions by inserting a lens to change the focal properties of the system as described in US Publication No. 20070291277. In addition to collecting data at different depths or locations, different scan patterns covering different transverse extents can be desired depending on the particular application.
Although imaging the retina in the posterior segment is concerned important for most major diseases of the eye, often it is desired to obtain OCT images on the cornea, which is in the anterior segment. One approach to this is to insert a diverging lens (or lenses) into the OCT optical train so as to form a virtual point source near the pupil conjugate. This results in a beam waist near the pupil of the subject. The power of the lens can be set so that the beam waist is on the cornea of a typical eye. Portions of the optical train are then moved along the optical axis by a typical eye length simultaneously with the addition of the lens, so as to quickly switch between retinal and a variety of corneal OCT imaging modalities. One type of add-on lens could be used for pachymetry measurements and narrows the field-of-view (FOV). Alternatively, sometimes a FOV wider than is available with the primary lens is desired which can be achieved by a different lens.
Precise knowledge of the optical train, including but not limited to the existence, identity, and location of optical components and their alignments, is an important aspect in ophthalmic optical coherence tomographic systems, where light is directed to an eye of a patient. It is desirable for such systems to operate in a multiplicity of modes, whereby the mode switching is performed with a high degree of automation. While it is desirable for all the adjustments to be made within the instrument, the use of add-on lenses to expand an OCT instrument's imaging capability, can require the operator to intervene. Switching from one OCT system imaging modality to another, with the concomitant change of optics, can introduce the chance for failure which could yield erroneous results (incorrect measurement or compromised signal strength). Thus there is a need for verification of the optical train prior to any use with a patient to insure that the correct optics were attached by the user and that the system makes any additional configuration adjustments necessitated by the addition of the add-on lens. Moreover, this verification needs also to be robust, meaning little or no chance for it to fail itself.
There have been several approaches to introducing such verification procedures into commercially available clinical products. In one approach, described in U.S. patent application Ser. No. 13/803,522, a diffused feature can be created by painting a small part or area of the lens white or gray. Alternatively, a small area of the housing where the lens is or lenses are held in place can be painted. A light from light source in the ocular lens housing illuminates the diffused feature from behind the lens and is imaged through a light mask onto the instrument's viewing camera. These approaches may have deficiencies in the adequacy and complexity of detection of lenses and their alignments.